1. Field of the Invention
The present invention relates to a liquid ejection head, a liquid ejection apparatus using same, and an image forming apparatus, and more particularly, to a nozzle arrangement structure of a liquid ejection head in which a plurality of ejection ports (nozzles) are arranged two-dimensionally at high density, and an image forming apparatus, such as an inkjet recording apparatus, which forms an image on a recording medium using the liquid ejection head.
2. Description of the Related Art
In the field of inkjet recording apparatuses, in order to print images of high quality at high speed, liquid ejection heads having a plurality of nozzles arranged two-dimensionally (so-called “matrix array heads”) have been proposed. FIG. 17 is a plan diagram showing a schematic view of an example of the composition of a matrix array head in the related art. The matrix array head 300 shown in FIG. 17 is a full line type head in which a plurality of nozzles 310 are arrayed two-dimensionally through a length corresponding to the full width of a print medium 302 in the direction (main scanning direction: the direction of arrow M) that is perpendicular to the conveyance direction of the print medium 302 (the sub-scanning direction: the direction of arrow S).
The pressure chambers 312 corresponding to the nozzles 310 are coupled to a common flow channel for ink supply (not shown) through independent supply ports (not shown), in such a manner that ink is filled into the pressure chambers 312 from the common flow channel. Furthermore, pressure generating elements (for example, piezoelectric elements) (not shown) are provided in the pressure chambers 312, and ink droplets can be ejected from the nozzles 310 by controlling the driving of the pressure generating elements in accordance with the print data. By controlling the ink ejection timings of the nozzles while conveying the print medium, it is possible to record a desired image on the print medium.
The matrix array head 300 in the related art has a structure in which a plurality of ink chamber units (liquid droplet ejection elements, each of which forms one recording element unit) 314, each comprising the nozzle 310, the pressure chamber 312 corresponding to the nozzle 310, the pressure generating element, and the like, are arranged in an oblique lattice configuration on the basis of a fixed arrangement pattern of a row direction aligned with the main scanning direction shown in FIG. 17 and an oblique column direction having a uniform angle α, which is not perpendicular to the main scanning direction. Taking the distance in the sub-scanning direction between the nozzle columns aligned in the main scanning direction on the nozzle surface 300A (sub-scanning direction nozzle pitch) as dNs, then the effective distance PN between the nozzles when projected to be aligned in the main scanning direction (the main scanning direction projected nozzle pitch) is dNs/tan α. By means of this composition, the distance between the dots that are mutually adjacent in the main scanning direction on the print medium 302 is narrowed, thereby improving the recording resolution.
By using the above-described matrix array head 300, it is possible to record an image over the whole surface of the print medium 302, by means of a single sub-scanning action (by performing just one operation of moving the print medium 302 relatively in the sub-scanning direction with respect to the matrix array head 300).
However, the matrix array head 300 in the related art, has a problem in that band-shaped density non-uniformity is liable to occur in the regions of the print medium 302 corresponding to the juncture sections between the nozzle columns (the juncture regions), due to error in the angle of the head in the direction of rotation in the plane of the head surface (error in the rotational position of the head when installed), due to skewed travel of the print medium 302, or the like.
This phenomenon is described with reference to FIG. 18. FIG. 18 shows a case where the head is disposed in a state where it is rotated slightly in the counter-clockwise direction within the plane parallel to the nozzle surface 300A, from the originally intended installation position (the designed reference position), namely, a case where the installed head is tilted with respect to the original main scanning direction. A relatively similar phenomenon also occurs, if the direction of conveyance of the print medium is tilted with respect to a correctly installed head (in the reference position), namely, in a case where the print medium is conveyed in a skewed or meandering fashion in the direction of the arrow S′ in FIG. 18.
As shown in FIG. 18, if the matrix array head 300 is installed with error in the rotational position (and/or the print medium is conveyed with a certain angle of inclination with respect to the sub-scanning direction due to skewed or meandering travel), then the distance Pm in the main scanning direction between the dots DA and DB formed on the print medium by droplets ejected from a pair of nozzles at the junction between the nozzle columns indicated by A and B in FIG. 18 (in the juncture region), is greater than the distance between other mutually adjacent dots in the main scanning direction (see the enlarged diagram in FIG. 19). Consequently, banding of a lower density occurs at a position on the print medium corresponding to the nozzle pair A and B in this juncture region. If the direction of rotation of the relative angular error between the matrix array head 300 and the print medium is the opposite direction, then the distance in the main scanning direction between the adjacent dots formed by droplets ejected onto the print medium by the nozzles in the juncture region is reduced, and therefore banding in which the recording density on the print medium becomes higher occurs correspondingly to the positions of the juncture regions.
Various methods have been proposed in order to reduce the perceivability of the density non-uniformity occurring at junctions (juncture regions) between nozzle columns (see Japanese Patent Application Publication Nos. 2004-167982, 2002-273878, and 2004-90504).
Japanese Patent Application Publication No. 2004-167982 discloses that nozzles are arranged in such a manner that sizes of the droplets ejected from the nozzles arranged in the sub-scanning direction are varied in an oscillating fashion, and that nozzles are arranged in such a manner that the positions of the nozzles in the main scanning direction are varied in an oscillating fashion. Accordingly, it is possible to reduce non-uniformity occurring at the same frequency as the juncture regions between nozzle columns in the matrix configuration. Furthermore, Japanese Patent Application Publication No. 2004-167982 discloses that non-uniformity is made to less perceivable by arranging nozzles in such a manner that the spatial frequency of the juncture regions of the matrix arrangement is a high frequency, which is not readily perceivable.
However, in this technology, it is not possible to reduce the error in the distance between dots occurring in the region where banding occurs, and hence there is a problem in that a large local difference in density occurs. Furthermore, since the dot diameters are intentionally made to be non-uniform, then the homogeneity of the print results is lost. In other words, this only allows the perceivability of the periodic non-uniformity at the juncture regions to be reduced by adding a new non-uniformity (variation) of a different frequency, by varying the dot sizes and varying the dot positions, in response to the original problem to be solved, namely, the periodic non-uniformity at the juncture regions.
Japanese Patent Application Publication No. 2002-273878 discloses a line type inkjet head in which nozzles are arranged in a column shape (linearly in one column or two columns) on a head chip, and a plurality of head chips are arranged in the line arrangement direction of the head on a single substrate in an oblique state where the nozzle arrangement direction of each head chip forms a prescribed angle with respect to the line arrangement direction. The technology disclosed in Japanese Patent Application Publication No. 2002-273878 aims to improve manufacturing accuracy and to reduce non-uniformity occurring at the junction regions between columns, by arranging the plurality of head chips on the single substrate.
However, this technology can reduce only the non-uniformity that occurs due to error in the nozzle arrangement, and it does not enable reduction of non-uniformity occurring due to inclination within the plane of the head, or skewed travel of the medium, or the like.
Japanese Patent Application Publication No. 2004-90504 discloses that the perceivability of non-uniformity can be reduced through altering the frequency of the non-uniformity by arranging nozzles in such a manner that droplets are ejected to form dots of a different diameter, between dots of the same diameter aligned in the main scanning direction on the print medium.
However, this technology requires a composition in which dots of different diameters can be ejected, and the like. Furthermore, since it simply changes the spatial frequency of the non-uniformity, there is large error in the dot pitch when viewed on a micro level, and hence there is a large difference between the actual print result and the ideal print result.